Different jumper cables may be used for interconnection between the RS232 interfaces of different serial data transmission apparatus. The installation of these jumper cables is complicated. The use of regular jumper cables is practical for intercommunication between two apparatus or for serial data transmission from one apparatus, which sends out data, to a plurality of apparatus, which receive data only. It is quite commonly in offices, factories, laboratories, etc., that serial data have to be transmitted from one serial data transmission apparatus to another, for example: the measured data from an instrument may have to be transmitted to a personal computer; a personal computer may have to send a file to a plotter or a remote apparatus through a modem; two or more personal computers may have to exchange files. When serial data transmission is needed among a variety of serial data transmission apparatus, the installation of different jumper cables often bothers the users.